Cannulated bone screws provided with an axial throughbore or cannula extending therethrough are known to allow use of guide wires that penetrate a bone at the desired implant site for the screw and guide the screw as it is being rotatably driven into the bone. One well known problem with bone screws is that as they start to go down in size in terms of the outer diameter of their threads, i.e., major diameter, the cannulas thereof likewise become very small so that only very thin guide wires can be used therewith. These thin guide wires are fairly flimsy and thus tend to bend and buckle, and walk on the surface of the bone when being inserted into the bone material. When this occurs, the location and/or angle of penetration varies from that which is desired making for inaccuracies in implanting the screw. In some instances, the screw will need to be backed out ruining the site and requiring identification of another implantation site, which can be difficult depending on the bone and bone location, and in any event is inconvenient to the doctor applying the screw into the bone.
Strength characteristics, such as the bending and torsional strength of a screw, are an important consideration with bone screws, particularly at small major diameter sizes thereof, e.g. 7.5 mm and less. Most failures of bone screws during insertion occur at the smaller screw sizes. More particularly, as the surgeon applies torque to the screw via a cannulated driver tool, the torsional forces can cause the screw to shear leaving only a lower portion that is partially driven into the bone material with the upper portion broken off from the remainder of the screw. At this point, additional medical intervention is necessary to remove or otherwise deal with the partially implanted screw portion and any broken off portions that may be loose at or around the implant site. Generally, with small screw sizes increasing the strength of the screws comes at the expense of having a larger cannula.
Similarly, the pull-out resistance of small screws can be compromised with increasing strength. The pull-out resistance is typically dependant on the purchase of the screw or its thread depth. With stronger, thicker-walled screws, the depth of the threads cannot be maximized for a particular size screw. In other words, if the screw cannula size is kept constant and the thread major diameter is not changed, the screw purchase will increase only with a generally corresponding decrease in screw strength.
Accordingly, there is a need for a cannulated bone screw particularly in small sizes thereof that allows it to be used with larger and firmer guide wires during implantation thereof. More particularly, small sizes of screws are needed that have an optimal combination of screw strength, cannula size, and thread purchase.